remote MX records/sending mail

How are you suppose to set exim up to check /etc/remote_domains for valid domain names, when verifying the mail from: on a new message? We have clients that use third party MX records and we modify their DNS record to point to those hosts. However mail still gets delivered locally to the server if they use their mail.domain.com as their SMTP server. Used to be able to just remove the domain from /etc/localdomains, but now exim uses /etc/localdomain to see if the domain name in the mail from is a valid domain name on the server (not arguing that feature, I like having that). Is there anyway to configure exim to also look at /etc/remote_domains for valid domain names, since /etc/remote_domains is where you are suppose to list domains that use 3rd party MX records.